A conventional exhaust gas boiler is made up of a steam/water cylinder and a convection part, most commonly water-pipe-structured, recovering thermal energy. The pipes may be smooth or ribbed, and they are usually horizontally or vertically disposed. The water/steam cylinder acts as the storage container for the liquid to be fed to the convection part and for the steam generated therein. In addition, the exhaust gas boiler comprises a frame, a heat-insulated housing, inlet and outlet openings for exhaust gas, the necessary valves, piping, possibly collector pipes, pumps, control devices, safety devices, and a control panel. Before being fed into the boiler the water is treated by using boiler-water treatment devices and chemicals.
Exhaust gas boilers have most commonly been used on ships and in diesel power stations, in which operational reliability is of a very high importance. Partly for this reason, the systems in use seem simple and outdated in their technology. Furthermore, especially for ships, classification institutions control closely the systems used, and extensive and thorough test runs are carried out on new constructions, even for reasons of maritime safety. In cumbersome, deficient and severe operating conditions, the simplest solutions with respect to both regulation and other functions have proved best in terms of operational reliability. In the exhaust gas boilers of steam-generating diesel engines or the like, power regulation has usually been implemented by causing a portion of the exhaust gases to bypass the boiler, so-called bypass regulation, or by condensing any excess steam with a cooling substance such as water, air or the like, so-called condensation regulation. On the basis of their operating mode, the boilers have convention ally been classified into free-circulation boilers, in which the circulation is effected by gravity and by temperature differences, and forced-circulation boilers, in which the circulation is effected by means of a pump or a corresponding device. In forced-circulation boilers, a throttle valve, most commonly situated on the pressure side of the pump, has been used for power regulation, so called throttle regulation.
For bypass regulation it has been necessary to construct for the exhaust gas boiler a bypass, which is difficult to implement as a construction and expensive, since the arrangement additionally requires a regulating damper by means of which the exhaust gas flows are directed into the exhaust gas boiler and to bypass it. In order for the regulation properties to be good, the regulating damper should be double-acting, i.e. it should regulate the flows both to the boiler and to the bypass The use of two separate dampers is an expensive option. In practice, the option of one regulating damper has often been settled for, at the expense of the regulating properties and flaw resistances. The regulating damper has to operate it hot and soiling conditions, and thus the operational reliability will suffer. If the bypass flow of the boiler is increased, the flow velocity of the exhaust gases in the convection part is reduced. From this there follows the adhering of exhaust-gas soot, oil and other solids to the surfaces of the convection part; this weakens heat exchange, and in the worst case causes a fire.
In the condensation regulation system there is no need for an exhaust gas bypass or a regulating damper. Instead, for dumping the excess steam coming from the exhaust gas boiler there is needed a condenser, water and steam pipes, pumps and/or blowers, control valves, and regulators. The condenser may be either water-cooled or air-cooled.
In addition to the costs of investment, in the condensation regulation system there are incurred operating costs from the use of the pumps and/or blowers, in addition to which the cooling water may in some conditions be expensive, or a warm climate may require the increasing of the heat exchange surface of the air coolers and of the blower efficiency. Running at a constant power will also increase the boiler water treatment costs and the apparatus size. It can be deemed to be a good feature of the condensation regulation system that the exhaust gases have the highest possible flow velocity in the convection part, thus reducing soiling.
In forced-circulation exhaust gas boilers the pipes are often in a horizontal plane, and the pipes in different horizontal planes are interconnected by curved pipe parts, circulation occurring from one plane to another. Thereby sufficiently long pipes are obtained for the high velocities of the forced circulation, and there is time for vaporization to take place. A forced-circulation exhaust gas boiler can also be coupled so that pipes in one and the same horizontal level are connected to one another. In throttle regulation of an exhaust gas boiler, the flow is reduced in order to reduce the boiler power. When the flow velocity decreases sufficiently, water will be left lying on the bottom of horizontally disposed pipes and will form boiler scale as it boils dry. In some cases there may form separate steam pockets which, when discharging, may damage the boiler. When a forced-circulation exhaust gas boiler which has been run dry is being started, there forms a strong thermal shock, which will cause high thermal stresses and will strain the boiler.
FI patent publication No. 64978 discloses an exhaust gas boiler wherein the heat exchange surfaces of the convection part are made up of pipe coils made of smooth pipe. The boiler is cylindrical and the smoke ducts of the convection part are circular as seen from above and from below. The space left inside the innermost coil is used as a bypass duct in which also the silencer is situated. The regulating damper is an top of the bypass, and when it is closed, all exhaust gases will flow into the convection part. When the regulating damper is open, a portion o f the exhaust gas flow will travel via the convection part. Soiling is reduced by a smooth pipe structure. A regulating range of 0-100% is not achieved with the construction according to the FI patent. Owing to the smooth pipes, the thermal surfaces remain small and the boiler will be heavy in high power categories. It is best suited for high exhaust gas temperatures, i.e. for rapid diesel engines.